add Genshin Impact Fandom event source

Add genshin-impact.fandom.com as a second source for Genshin Impact,
which has been fixture-backed in CI because Game8 CloudFront-blocks the
Actions runner with 202s.

- Add fifth Fandom template parser in src/ingest/parsers/fandom.ts
  extracting titles from caption display text, stripping dated subpage
  suffixes, and checking currency via latestBoundaryMs.
- Register genshin-fandom-events in src/ingest/adapters/index.ts and
  set genshin-game8-events priority: 10 so existing event IDs survive
  near-match ties, preventing localStorage completion marks from
  orphaning.
- Explicitly sort adapter groups by priority descending in
  scripts/build-feed.ts to align with parseGame.
- Pin fixtures and add test coverage in test/adapters/game8.test.ts.
- Update AGENTS.md, README.md, docs/INGESTION.md, and docs/SOURCES.md.

scripts/build-feed.ts

@@ -58,7 +58,7 @@ async function documentFor(adapterId: string, game: GameId) {
 }
 
 const now = new Date().toISOString();
-const byGame = new Map<GameId, GachaEvent[][]>();
+const byGame = new Map<GameId, Array<{ priority: number; events: GachaEvent[] }>>();
 const sources: SourceHealth[] = [];
 
 for (const adapter of ADAPTERS) {
@@ -76,7 +76,7 @@ for (const adapter of ADAPTERS) {
   const { parsedCount } = health;
 
   const groups = byGame.get(adapter.game) ?? [];
-  groups.push(events);
+  groups.push({ priority: adapter.priority, events });
   byGame.set(adapter.game, groups);
 
   sources.push(health);
@@ -96,7 +96,10 @@ for (const adapter of ADAPTERS) {
 const events: GachaEvent[] = [];
 let conflictCount = 0;
 for (const [, groups] of byGame) {
-  const merged = mergeEvents(groups);
+  const sorted = groups
+    .sort((a, b) => b.priority - a.priority)
+    .map((g) => g.events);
+  const merged = mergeEvents(sorted);
   events.push(...merged.events);
   conflictCount += merged.conflicts.length;
   for (const c of merged.conflicts) {